The present invention relates to an electrolyte solution for electrolytic metal plating or, more particularly, to an aqueous electrolyte solution as a bath for electrolytic metal plating in the manufacturing process of magnetic recording media by vertical-mode magnetization suitable for high-density magnetic recording such as fixed magnetic discs, floppy discs and the like.
Along with the rapid progress in the computer technology in recent years, a great progress has been marked also in the data recording devices using, for example, magnetic discs for which it is eagerly desired to more and more increase the density of magnetic recording and the velocity of recording. One of the recently highlighted improvements in this regard is the conversion of the direction of magnetization from the conventional horizontal direction, i.e. within the plane of the magnetic layer, to the vertical direction, i.e. perpendicular to the plane of the magnetic layer. A great future progress can be forseen by this means in the increase of the density and velocity of magnetic recording on a magnetic recording medium.
The magnetization mode of the vertical magnetic recording consists in the direction of the magnetization which is perpendicular to the plane of the substrate or, consequently, to the plane of the magnetic layer on the substrate surface. This magnetization mode permits a high-density recording since the demagnetization field approximates zero even with a very short wavelength of recording. This means that the thickness of the magnetic layer may not be so small as compared to the horizontal mode of the magnetic recording in which high-density recording can be performed only with an extremely small thickness of the magnetic layer. In the vertical mode of the magnetization recording, moreover, very sharp magnetization reversal can be obtained in the magnetic recording medium or the magnetic domains because the demagnetization field scarcely acts on the areas of magnetization reversal contributing to an increase of the residual magnetic moment per bit. Therefore, the vertical mode of magnetization is advantageous in principle with a possibility of greatly improving the S/N ratio in the output of the recorded information to facilitate a high-density recording. It is important that the magnetic layer for such a magnetization mode should have an easily magnetizable axis in the direction perpendicular to the plane of the layer. This means that, assuming that the magnetic layer is formed of metallic cobalt, the easily magnetizable axis of the microcrystallites of .alpha.-Co should be oriented perpendicularly to the plane of the magnetic layer. As is known, a thin magnetic layer of metallic cobalt on a substrate surface can be formed by several methods including the methods of vacuum vapor deposition, high-frequency sputtering and plating, the former two being performed in vacuum and the last being performed in an open system.
When an anisotropic magnetic field is applied to a magnetic layer on a substrate in a direction perpendicular to the plane of the layer, assuming that the cobalt-based magnetic layer is added with a dopant element such as nickel, manganese and chromium with an object to improve the magnetic properties by decreasing the saturation magnetization, areas of magnetization reversal are almost free from the influence of the demagnetization field so that the saturation magnetization is increased within the magnetic layer to satisfy the requirement for a vertical-mode magnetic recording medium that the saturation magnetization should exceed the demagnetization field.
It is generally understood that an ideally improved magnetic recording medium can be obtained by using chromium as a dopant element of cobalt when the magnetic layer is formed by the method of vacuum vapor deposition or high-frequency sputtering carried out in vacuum. These methods, however, are not quite feasible as an industrial process due to the relatively high costs because the process is complicate and lengthy. On the other hand, no effective dopant element is known for cobalt-based magnetic recording layer formed by the method of plating carried out in an open system despite the eager desire to prepare a cobalt-based magnet-ic layer by the inexpensive plating method.
In view of the above mentioned problem, the inventors have previously proposed an improved electrolyte solution as a bath for the electrolytic plating of a cobalt-based alloy (see Japanese Patent Kokai No. 61-179891). The electrolyte solution here proposed contains ions of cobalt, manganese and nickel as the principal metallic ingredients together with a reducing agent and buffering and controlling agents of pH. Although the magnetic layer obtained by using this electrolyte solution as the bath for electrolytic plating is indeed excellent in connection with the reproducibility of the metallic composition and magnetic performance suitable for a high-density magnetic recording medium, it is still not quite satisfactory as a magnetic recording layer for the vertical-mode magnetization.